1. Field of the Invention
The present invention relates to an elastic wave device which repeatedly reflects an elastic wave between an input side electrode and an output side electrode device.
2. Description of the Related Art
A SAW (surface acoustic wave) filter being an elastic wave device is used, for instance, in the transmitting and receiving unit of a portable terminal, and there is a device using a resonant single-phase unidirectional transducers (RSPUDT) as its example. FIG. 10 shows the structure of a SAW filter of this kind, in which 11 is a piezoelectric substrate. The piezoelectric substrate 11 includes an input side resonant single-phase unidirectional transducers (RSPUDT) 12, an output side resonant single-phase unidirectional transducers (RSPUDT) 13 and a shield electrode 14. The respective resonant single-phase unidirectional transducers 12 and 13 include bus bars 15a and 15b, a number of exciting interdigital transducers 16 for reflecting the SAW and a number of dummy electrodes 17 which are shorter than the interdigital transducer 16 and not contributing to the excitation. The role of the dummy electrode 17 will be explained later. In general, the one-way electrode shows a wide filter characteristic being the characteristic of a transversal type filter when the reflection of the electrode is weak, while when the reflection is stronger, its internal reflection becomes stronger, and the electrode shows narrow filter characteristics close to the characteristics of a resonator type filter.
In the resonant single-phase unidirectional transducers which has a strong internal reflection, the reflection plane confining SAW energy exists in the propagation direction of the SAW (on the extensional direction of the bus bar). The reflection plane exists in the vicinity of the center in the propagation direction of the SAW in respective interdigital transducers 16 on the input side and the output side. The one-way electrodes 12 and 13 are configured such that by utilizing multiple reflections between the reflection planes so that the SAW can be strongly propagated only in one direction from the input side to the output side. In addition, the width and the distance of arrangement of the interdigital transducer 16 are appropriately adjusted so that the SAW is propagated in this way.
In such a SAW filter 1 having the resonant single-phase unidirectional transducers 12 and 13, of which internal reflection is strong as described above, the SAW energy is concentrated on the reflection planes, which causes the following problems. For instance, the surface wave 10a which propagates through an electrode finger formation area 18 surrounded by a chain line in the figure as shown in FIG. 11A originally advances in a rectangular shaped wave, however, it sometimes happens that as the wave reflects and propagates, its waveform is deformed, and the wave is incident on a boundary between the electrode finger formation area 18 and the bus bars 15a, 15b. When the electrode finger formation area 18 is taken as a medium, the bus bars 15a, 15b and the electrode finger formation area 18 are different in medium from each other where the medium is concerned, and due to the difference in medium, the wave incident on the boundary surface causes reflection. Then, the lateral mode waves run high vertically distributing relative to the propagating direction of the SAW for which the electrode finger formation area 18 serves as a wave guide. The rectangular wave 10a becomes a sine wave 10b being a primary mode (ordinary mode) as shown in FIG. 11B, and further, a higher mode wave in a lateral mode such as the wave 10c in a tertiary mode, a fifth mode, a seventh mode or the like (these are not shown) appears.
When only the sine wave 10b moves forward through the electrode finger formation area 18, the SAW filter 1 shows a filter characteristic shown by a solid line in FIG. 12A, and the above-described respective higher mode waves have propagation areas where the signs of the electric charges induced in the interdigital transducers 16 are opposite to each other. In other words, when the higher mode waves propagate over the electrode finger formation area 18, for instance, a positive charge is induced in a certain interdigital transducer 16, and a negative charge is induced at another interdigital transducer 16, so that the balance of the energy of the electric charge in the electrode finger formation area 18 is disturbed. Then, the energy of the electric charge leaks out from the center side of the electrode finger formation area 18 into the vicinity of the bus bar 15a and 15b respectively, which causes an influence in the excitation of the interdigital transducer 16, so that an amplitude characteristic exhibiting the peak on the higher band area than the central frequency of the sine wave 10b appears, for instance, shown by a chain line and a two-dot chain line in FIG. 12A. When the sine wave 10b and higher mode waves repeat reflection in the electrode finger formation area 18 and resonate so as to be in a resonance mode, the characteristics of the respective waves are added to become a filter characteristic shown in FIG. 12B. In other words, the SAW filter 1 is, as shown by the area surrounded by the chain line, to possess a characteristic in which the spuriousness appears in a damping band on the higher band side than the pass band. As above, generation of higher mode waves in a lateral mode and occurrence of resonance are verified in a simulation study in non-Patent Document 1.
Then, in the SAW filter 1, the length of the interdigital transducer 16 is adjusted and the dummy electrode 17 is disposed, and further, the ratio of the aperture (W) of the electrode finger to the width of opening (WO) of the electrode finger from the respective bus bars 15a and 15b is adjusted. By this adjustment, when the higher mode wave is propagated, the electric charge opposite in sign to the electric charge induced by the interdigital transducer 16 existing on the extension is induced in the dummy electrode 17, so that the positive and negative charges in the electrode finger formation area 18 are cancelled so as to be close to zero. Thus, by preventing lowering of the excitation of the interdigital transducer 16, reduction of the spuriousness is expected. Such an electrode configuration (W/WO configuration) is described in Patent Document 1.
In such a W/WO configuration, when the width of crossing W is large, the spuriousness in the lateral mode is likely to occur in a plurality of cases, though it is possible to reduce the occurrence of the spuriousness by narrowing the aperture W, there arises problems that the desired filter characteristic cannot be obtained due to the high impedance and widening of the band width. Thus, since there is limitation on controlling the W/WO ratio, it is still insufficient to reduce the energy of the electric charge generated by the higher mode waves, and the energy still remaining in a high level leaks out into the vicinity of the bus bars 15a and 15b from the central side in the direction of distribution of the lateral mode, resulting in generation of the spuriousness.
Patent Document 2 shows a SAW filter in a configuration such that a damper is applied on the bus bar part parallel to the direction of propagation of waves. The SAW filter, however, is a transversal type filter using an apodized electrode, and has a configuration in which there is no occurrence of internal reflection in the electrode finger formation area. The damper is used for the purpose of preventing reflecting waves from a tip terminal face parallel to the direction of propagation of the SAW, and not for the purpose of reducing the spuriousness in a higher lateral mode. Therefore, the invention described in Patent Document 2 is not to solve the problems which the above-described spuriousness creates. Patent Document 3 describes a SAW filter which has a damper applied on a bus bar of an input/output electrode thereof and reduces generation of spuriousness by preventing unification of bulk waves and pseudo surface acoustic waves. However, there is no reference to the position of the damper on the bus bar.
[Patent Document 1] Japanese Patent Application Laid-open No. Hei 9-260996
[Patent Document 2] Japanese Patent Application Laid-open No. Hei 9-205341
[Patent Document 3] Japanese Patent Application Laid-open No. Hei 6-232682
[non-Patent Document 1] Marc Solal et al. Design Modeling and Visualization of Low Transverse Modes R-SPUDT Devices 2006 IEEE Ultrasonics Symposium